1. Field of the Invention
The present invention relates to a display control apparatus. More specifically, the present invention relates to an improvement in a display control apparatus for controlling a display operation of a display which comprises a plurality of picture elements such as a dot display, a CRT display, and the like.
2. Description of the Prior Art
A display has been employed most typically in various types of electronic equipment in order to display information. Typical examples of such displays which are well-known are a dot display, a CRT display and the like. Any of such displays has a screen formed with a group of a plurality of minimum display units, i.e. picture elements or dots, whereby desired characters are displayed as a combination of an enabled or disabled state of each of the picture elements. A conventional display control apparatus for controlling a display operation of such display comprises a memory for receiving display data from an external processing unit, such as a central processing unit of a microprocessor, thereby to store the same, and functions to read the stored data in the memory as a function of a synchronizing signal. The respective picture elements in the display are selectively driven for display as a function of the read data. Meanwhile, renewal of the display data, i.e. renewal of the stored data in the memory must be performed while a display operation by the display is interrupted in the course thereof and hence a flickering phenomenon was caused thereby. In order to prevent the same, a conventional display control apparatus was adapted to renew the data in the memory during a blanking period of the display. However, such a display control apparatus necessitates a detecting circuit for detecting a blanking period from the synchronizing signal and hence involves a disadvantage that the circuit configuration becomes complicated and expensive. Furthermore, since renewal of the display data stored in the memory is limited only to the above described display blanking period according to a conventional display control apparatus, it becomes impossible to renew the display data during a period shorter than a period required for displaying all of one frame of the display and hence another disadvantage is also involved that such a conventional display control apparatus can not be employed in a display requiring a change of the display data during a relatively short period.
The above described display is utilized in some cases such that fixed data is displayed in a certain region of the display screen and changeable data is displayed in other regions of the display screen. The fixed data could be units of current time information, such as day, hour, minute, framing of the screen, headline data of various kinds of data, lining in graphs and other fixed display patterns. On the other hand, the changeable data could be data of the current time information, various kinds of numerals or symbols or characters, and lines of graphs and the like.
Meanwhile, a conventional display control apparatus for displaying both the fixed data and the changeable data in a display was structured such that a display data memory is provided which has a plurality of bits corresponding to a plurality of picture elements on the screen of the display and display data entered from an external processing unit such as a microprocessor is stored in the display data memory and the data in the memory is sequentially read out by a read control circuit in synchronism with addressing of the display coordinate positions of the display, whereby the data is displayed on the display. However, since the fixed data must be displayed at a predetermined position on the screen, the data need be read from the memory and the same need be displayed at a predetermined period. On the other hand, since the changeable data is stored in the memory and is read out therefrom, renewal of the data in the memory is required. Necessity of storing such changeable data in a renewing manner is different depending on the kinds of the data to be displayed but such necessity arises frequently and therefore frequent renewal of the data in the memory necessitates a considerable processing time period. However, the reading out of the changeable data is performed during a time period after completion of renewal storing until reading of the fixed data. Therefore, the reading time period of the changeable data changes depending on the amount of the changeable data and in an extreme case it could happen that the reading time period of the changeable data extends to the reading time period of the fixed data. Therefore, disadvantages were involved that the changeable data and the fixed data could be displayed in an overlapped manner and a flickering phenomenon is liable to be caused.